Boundary acoustic wave device and method of manufacturing same

ABSTRACT

In the boundary acoustic wave device, an IDT electrode, a first dielectric layer, and a second dielectric layer are provided on a piezoelectric substrate. The first dielectric layer is made of a deposited film. A thickness of the IDT electrode is about 10% or more of λ. A difference between a height of the first dielectric layer, measured from an upper surface of the piezoelectric substrate, above a center of an electrode finger of the IDT electrode and a height of the first dielectric layer, measured from the upper surface of the piezoelectric substrate, above a center of a gap between adjacent electrode fingers, i.e., a magnitude of unevenness in an upper surface of the first dielectric layer, is about 5% or less of λ.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a boundary acoustic wave devicepreferably for use in a resonator, a band filter, etc., and to a methodof manufacturing such a boundary acoustic wave device. Moreparticularly, the present invention relates to a boundary acoustic wavedevice including a so-called three-medium structure in which first andsecond dielectric layers are stacked on a piezoelectric substrate, andto a manufacturing method thereof.

2. Description of the Related Art

Conventionally, in communication systems such as cellular phones,surface acoustic wave devices have been widely used as resonators andband filters. Also, attention has been focused on boundary acoustic wavedevices instead of surface acoustic wave devices because the former doesnot require a package structure having a cavity. For that reason,boundary acoustic wave devices of various structures have been proposed.

WO2005/093949 discloses a boundary acoustic wave device 101 illustratedin a sectional view of FIG. 9. The boundary acoustic wave device 101 isa boundary acoustic wave device of a three-medium structure. In theboundary acoustic wave device 101, first and second dielectric layers103 and 104 are stacked on a piezoelectric substrate 102. An electrodestructure including IDT electrodes 105 is formed at an interface betweenthe piezoelectric substrate 102 and the first dielectric layer 103.

When manufacturing the boundary acoustic wave device 101, firstly, thepiezoelectric substrate 102 is prepared. The electrode structureincluding the IDT electrodes 105 is then formed on the piezoelectricsubstrate 102. The first dielectric layer 103 is then formed bymagnetron sputtering. In this stage, the frequency, the acousticvelocity of a boundary acoustic wave, etc., are adjusted by adjusting afilm thickness of the first dielectric layer 103. After the adjustment,the second dielectric layer 104 is formed.

The first dielectric layer 103 can also be formed by a method of bondinga dielectric wafer, which is prepared separately, instead of magnetronsputtering. However, it is difficult to uniformly bond the dielectricwafer. Further, there is a risk that the first dielectric layer 103 maybe peeled off when a laminate body obtained as a mother substrate iscut. On the other hand, the first dielectric layer 103 can be easily andreliably formed by the manufacturing method described in WO2005/093949because it utilizes the magnetron sputtering.

With the manufacturing method described in WO2005/093949, however, evenwhen the frequency and the acoustic velocity of a boundary acoustic waveare adjusted before formation of the second dielectric layer 104,frequency characteristics, such as a resonant frequency and a centerfrequency, tend to vary in the manufactured boundary acoustic wavedevice 101 because the second dielectric layer 104 is formed after theadjustment.

SUMMARY OF THE INVENTION

In order to overcome the above-mentioned problems in the related art,preferred embodiments of the present invention provide a boundaryacoustic wave device and a method of manufacturing a boundary acousticwave device, in which a first dielectric layer can be easily andreliably formed, and in which a variation in frequency characteristicsof the finally obtained boundary acoustic wave device is significantlyreduced and minimized.

A boundary acoustic wave device according to a preferred embodiment ofthe present invention includes a piezoelectric substrate, an IDTelectrode provided on the piezoelectric substrate, a first dielectriclayer arranged to cover the IDT electrode and made of a deposited film,and a second dielectric layer disposed on the first dielectric layer.Further, assuming a wavelength of a boundary acoustic wave to be λ, anormalized film thickness of the IDT electrode is about 10% or more ofλ, and a difference between a height of the first dielectric layer,measured from an upper surface of the piezoelectric substrate, above acenter of an electrode finger of the IDT electrode and a height of thefirst dielectric layer, measured from the upper surface of thepiezoelectric substrate, above a center of a gap between adjacentelectrode fingers is about 5% or less of the wavelength λ.

In one particular aspect of the boundary acoustic wave device accordingto a preferred embodiment of the present invention, the first dielectriclayer preferably is the deposited film formed by bias sputtering, forexample. In that case, flatness of an upper surface of the firstdielectric layer can be increased.

In another particular aspect of the boundary acoustic wave deviceaccording to a preferred embodiment of the present invention, the uppersurface of the first dielectric layer is flattened. In that case, asurface of the second dielectric layer formed on the first dielectriclayer can be made flatter.

In still another particular aspect of the boundary acoustic wave deviceaccording to a preferred embodiment of the present invention, thepiezoelectric substrate preferably is made of LiNbO₃ or LiTaO₃, and thefirst dielectric layer preferably is made of silicon oxide. In thatcase, an absolute value of the temperature coefficient of frequency(TCF) in the boundary acoustic wave device can be reduced.

In still another particular aspect of the boundary acoustic wave deviceaccording to a preferred embodiment of the present invention, the seconddielectric layer preferably is made of at least one kind of dielectricmaterial selected from a group consisting of silicon nitride, siliconoxynitride, aluminum nitride, aluminum oxide, and silicon. In that case,since the acoustic velocity in the second dielectric layer is higherthan that in silicon oxide, a loss can be reduced due to the waveguideeffect.

In still another particular aspect of the boundary acoustic wave deviceaccording to a preferred embodiment of the present invention, the IDTelectrode is primarily formed of an electrode layer made of at least onekind of metal selected from a group consisting of Au, Ag, Cu, Pt, Ta, W,Ni, Fe, Cr, Mo, Ti, and an alloy containing one of those metals as amain component. In that case, by using, as the material of the IDTelectrode, a metal having a relatively higher density than that of thefirst dielectric layer, the waveguide effect can be enhanced and theloss can be further reduced.

According to another preferred embodiment of the present invention, amethod of manufacturing a boundary acoustic wave device includes thesteps of forming an IDT electrode on a piezoelectric substrate,depositing a dielectric material on the piezoelectric substrate to coverthe IDT electrode by a deposition method, thereby forming a firstdielectric layer, and forming a second dielectric layer on the firstdielectric layer.

In one particular aspect of the method of manufacturing the boundaryacoustic wave device according to a preferred embodiment of the presentinvention, the deposition method is bias sputtering. In that case,flatness of the upper surface of the first dielectric layer can beincreased.

In another particular aspect of the method of manufacturing the boundaryacoustic wave device according to a preferred embodiment of the presentinvention, the method further includes the step of flattening a surfaceof the first dielectric layer after forming the first dielectric layerby the deposition method. In that case, since the surface of the firstdielectric layer can be made flatter, unevenness in the surface of thesecond dielectric layer can be made smaller.

In still another particular aspect of the method of manufacturing theboundary acoustic wave device according to a preferred embodiment of thepresent invention, the step of flattening the upper surface of the firstdielectric layer is carried out by at least one process selected from agroup consisting of milling, dry etching, CMP, and etching-back of thesurface of the first dielectric layer. In that case, the upper surfaceof the first dielectric layer can be more reliably and easily flattened.

With the boundary acoustic wave device and the method of manufacturingthe same according to various preferred embodiments of the presentinvention, the film thickness of the IDT electrode preferably is aslarge as about 10% or more of the wavelength λ of the boundary acousticwave, while the difference between the height of the first dielectriclayer above the center of an electrode finger of the IDT electrode andthe height of the first dielectric layer above the center of a gapbetween the adjacent electrode fingers is preferably about 5% or less ofλ. Therefore, variations in frequency characteristics of the boundaryacoustic wave device are significantly reduced and minimized. As aresult, a boundary acoustic wave resonator, a boundary acoustic wavefilter, and other similar devices, etc., can be produced to have a smallvariation in frequency characteristics, such as a center frequency and aresonant frequency.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic plan view illustrating an electrode structure ofa boundary acoustic wave device according to one preferred embodiment ofthe present invention, FIG. 1B is a front sectional view of the boundaryacoustic wave device, and FIG. 1C is a partially-cut front sectionalview illustrating, in an enlarged scale, a principal portion of theboundary acoustic wave device.

FIG. 2 is an electron microscope photo to explain voids generated when aSiO₂ film is formed by RF magnetron sputtering.

FIG. 3 is a graph plotting the relationship between a film thickness ofan IDT electrode and an impedance ratio of a resonator when the SiO₂film is formed by bias sputtering and ordinary RF magnetron sputtering.

FIGS. 4A to 4C are each a schematic sectional view illustrating thestate of an interface between the SiO₂ film and a SiN film, the SiO₂film being formed by the bias sputtering at a bias voltage of V1, V2 orV3, respectively.

FIG. 5 is a schematic front sectional view to explain the magnitude ofunevenness in an upper surface of the SiO₂ film formed on the IDTelectrode.

FIG. 6 is a graph plotting the relationship between the magnitude of thebias voltage and the magnitude of unevenness in the upper surface of theSiO₂ film when the bias sputtering is carried out.

FIG. 7 is a graph plotting changes of the resonant frequency withrespect to the magnitude of unevenness in the upper surface of the SiO₂film after formation of the SiO₂ film and after formation of the SiNfilm.

FIG. 8 is a graph plotting the relationship between the magnitude ofunevenness in the upper surface of the SiO₂ film and a frequency changein a finally-obtained boundary acoustic wave device.

FIG. 9 is a front sectional view to explain a boundary acoustic wavedevice of the related art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be clarified by the following description ofpreferred embodiments of the present invention with reference to thedrawings.

FIG. 1A is a schematic plan view illustrating an electrode structure ofa boundary acoustic wave device according to one preferred embodiment ofthe present invention, FIG. 1B is a front sectional view of the boundaryacoustic wave device, and FIG. 1C is a partially-cut front sectionalview illustrating, in an enlarged scale, a principal portion of theboundary acoustic wave device.

A boundary acoustic wave device 1 includes a piezoelectric substrate 2.First and second dielectric layers 3 and 4 are stacked on thepiezoelectric substrate 2 in this order. Thus, the boundary acousticwave device 1 is a boundary acoustic wave device including a so-calledthree-medium structure.

In this preferred embodiment, a 25°-rotated Y cut LiNbO₃ single-crystalsubstrate is preferably used as the piezoelectric substrate 2, forexample. The first dielectric layer 3 is made of SiO₂, for example. Thesecond dielectric layer 4 is made of SiN, for example.

As described later, the first dielectric layer 3 is preferably formed bybias sputtering, for example. The second dielectric layer 4 made of SiNis preferably formed by RF magnetron sputtering, for example. In otherwords, the first and second dielectric layers 3 and 4 are preferablyformed by deposition methods.

An electrode structure including an IDT electrode 5 and reflectors 6 and7 disposed on both sides of the IDT electrode 5 is provided on an uppersurface of the piezoelectric substrate 2. The IDT electrode 5 includes aplurality of electrode fingers 5 a. The IDT electrode 5 and thereflectors 6 and 7 are each formed by stacking a plurality of metalfilms. In this preferred embodiment, a Ti film/Pt film/Ti film/AlCufilm/Ti film/Pt film/Ti film are successively stacked in this order fromthe top.

The Ti film between the AlCu film and the Pt film serves as not only abarrier layer to prevent inter-diffusion of metal atoms between themetal films on both sides thereof, but also as an adhesive layer toincrease adhesion to the metal films on both sides thereof. The Ti filmbetween each of the first dielectric layer and the piezoelectricsubstrate and the Pt film serves as an adhesive layer.

Accordingly, the film thickness of the Ti film serving as the adhesivelayer and/or the barrier layer is preferably set smaller than that ofeach of the Pt film and the AlCu film. Thus, the IDT electrode 5includes the Pt film and the AlCu film as main electrode layers.

In FIG. 1B, an upper surface of the first dielectric layer 3 isillustrated as being flat. However, when the first dielectric layer 3 isformed by the deposition method such as the bias sputtering, the uppersurface of the first dielectric layer 3 usually includes unevenness.

The boundary acoustic wave device 1 of this preferred embodimentpreferably is a 1-port boundary acoustic wave resonator including theIDT electrode 5 and the reflectors 6 and 7.

In the boundary acoustic wave device 1, because the acoustic velocity inthe second dielectric layer 4 is higher than that in the firstdielectric layer 3, a boundary acoustic wave is substantially confinedwithin the first dielectric layer due to the waveguide effect. As aresult, a loss can be reduced.

When the first dielectric layer 3 is formed by the deposition methodincluding the sputtering, a material forming the first dielectric layer3 is uniformly deposited on the piezoelectric substrate 2. Therefore,the film thickness of the first dielectric layer 3 is substantiallyuniform over an entire region. Hence, a height of the upper surface ofthe first dielectric layer 3 is increased in portions where the IDTelectrode 5 and reflectors 6 and 7 are present, thus causing unevennessin the upper surface of the first dielectric layer 3.

In this preferred embodiment, however, as illustrated in FIG. 1C, adifference between a height H1 of the first dielectric layer 3 above acenter of the electrode finger of the IDT electrode 5 and a height H2 ofthe first dielectric layer 3 above a center of the gap between theelectrode fingers is preferably set to be about 5% or less of λ, forexample. Stated another way, even when the thickness of the IDTelectrode 5 is set as large as about 10% or more of λ, the unevenness inthe upper surface of the first dielectric layer 3 is held relativelysmall. Here, the height H1 and the height H2 are each defined as a valuemeasured from an upper surface 2 a of the piezoelectric substrate 2 inthe height direction. Further, λ denotes a wavelength of the boundaryacoustic wave.

Since the difference between the height H1 and the height H2 ispreferably about 5% or less of λ in the boundary acoustic wave device 1,a variation in frequency characteristics, i.e., a frequency tolerance,can be kept small in the boundary acoustic wave device 1. That pointwill be described in more detail below.

Conventionally, in a boundary acoustic wave device, reducing theconductor resistance of an IDT electrode has been tried to reduce aloss. For example, it has been tried to increase the film thickness ofthe IDT electrode, or to use an electrode formed by stacking an Al film,which has a low conductor resistance, on a Pt film. However, when such amultilayer metal film is used, a total film thickness of the IDTelectrode is increased. Consequently, when a dielectric layer is formedso as to cover the IDT electrode by the deposition method, a leveldifference tends to occur in an upper surface of the dielectric layerbetween a portion where the IDT electrode 5 is present below and aportion where the IDT electrode is not present below.

The inventors of this application have discovered that theabove-described problem of increasing the frequency tolerance in theboundary acoustic wave device of the three-medium structure isattributable to the magnitude of unevenness in the surface of the firstdielectric layer 3. In preferred embodiments of the present invention,since the unevenness in the upper surface of the first dielectric layer3, i.e., the level difference between an uppermost portion and alowermost portion of the first dielectric layer, is preferably set to beabout 5% or less of λ as described above, the frequency tolerance can beheld very small. Particularly, even when, for the purpose of reducingthe resistance of the IDT electrode 5, the thickness of the IDTelectrode is set as large as about 10% or more of the wavelength λ byincreasing the thickness of the IDT electrode 5 or by forming the IDTelectrode 5 with the use of the multilayer metal film, the frequencytolerance can be held very small.

That point will be described below based on a specific experimentalexample.

A non-limiting example of the boundary acoustic wave device 1 of theabove-described preferred embodiment was fabricated as follows. Thepiezoelectric substrate 2 made of 25°-rotated Y cut LiNbO₃ was prepared.The IDT electrode 5 and the reflectors 6 and 7 were formed on the uppersurface of the piezoelectric substrate by an electron beam evaporationmethod.

In this experimental example, prior to forming the IDT electrode 5 andthe reflectors 6 and 7, a Ta₂O₅ film was formed as an underlying film onthe upper surface of the piezoelectric substrate. The underlying film isnot always required. The formation of the underlying film contributes toincreasing adhesion of the IDT electrode 5, etc., to the piezoelectricsubstrate 2.

The IDT electrode 5 and the reflectors 6 and 7 were each formed as theabove-mentioned multilayer metal film. Respective thicknesses of themetal films were as follows.

In the IDT electrode 5, the wavelength λ was set to about 1.9 μm, a dutyratio was set to about 0.50, the number of pairs of the electrodefingers was set to 60, and an opening length, i.e., a distance betweenopposing bus bars, was set to about 30λ, for example. The number of theelectrode fingers in each of the reflectors 6 and 7 was set to 51.

Ti film: about 10 nm, i.e., about 0.5% of the wavelength λ.

Thickness of each Pt film: about 31 nm, i.e., about 1.6% of thewavelength λ.

AlCu film: made of an alloy containing Al as a main component with a Cuconcentration of about 1% by weight, and having a film thickness ofabout 300 nm, i.e., about 16% in terms of ratio to the wavelength.

Accordingly, the total thickness of the IDT electrode 5 was given byabout 0.5%×4+about 1.6%×2+about 16%=about 21.2% of the wavelength λ.

Thickness of Ta₂O₅ film: about 12 nm, i.e., about 0.6% of the wavelengthλ.

Next, as the first dielectric layer 3, a SiO₂ film was formed by thebias sputtering. The thickness of the SiO₂ film was set to about 712 nm,i.e., about 38% of λ.

Next, as the second dielectric layer 4, a SiN film was formed with athickness of about 2000 nm, i.e., about 105% in terms of ratio to thewavelength, by the RF magnetron sputtering.

Problems caused when fabricating a boundary acoustic wave device of thethree-medium structure likewise in accordance with the related-artmanufacturing method will be described below.

In trying to reduce the conductor resistance by increasing the thicknessof the IDT electrode, as described above, when SiO₂ serving as the firstdielectric layer, etc. are formed by the deposition method such as theRF magnetron sputtering, the unevenness of the surface of the SiO₂ filmis increased. Moreover, as seen from an electron microscope photo ofFIG. 2, a void may be generated in the SiO₂ above a center of a gapbetween the electrode fingers of the IDT electrode. Such a phenomenon ispresumably attributable to the fact that the SiO₂ films are graduallydeposited to cover adjacent finger electrodes and are contacted witheach other while forming a void above the center of the gap, thusforming an integral film. As a result, an impedance ratio in a boundaryacoustic wave resonator, i.e., a ratio of impedance at the anti-resonantfrequency to impedance at the resonant frequency, is reduced anddegraded. That point is illustrated in FIG. 3.

Marks ⋄ in FIG. 3 represent changes in the impedance ratio when SiO₂ isformed as the first dielectric layer 3 by the RF magnetron sputteringand a film thickness of the SiO₂ is gradually increased. As is apparentfrom FIG. 3, when the film thickness of the IDT electrode exceeds about10% in terms of ratio to the wavelength, the impedance ratio is quicklydropped from about 60 dB to about 50 dB, for example.

On the other hand, marks ◯ in FIG. 3 represent the result obtained whenthe film thickness of the IDT electrode is changed in this preferredembodiment in which the first dielectric layer 3 is formed by the biassputtering. As is apparent from FIG. 3, even when the film thickness ofthe IDT electrode is increased to about 25% in excess of about 10%, theimpedance ratio is not reduced. It is hence understood that, even whenthe total film thickness of the IDT electrode is increased byadditionally stacking a metal film having a low conductor resistance,e.g., an Al film, the impedance ratio is hard to degrade.

Meanwhile, in the boundary acoustic wave device of the three-mediumstructure, the frequency varies depending on the film thickness of thefirst dielectric layer 3. Therefore, the frequency can be adjusted byadjusting the film thickness of the first dielectric layer 3. Thatfrequency adjustment can be performed by a film-thickness adjustingmethod, such as milling. In this preferred embodiment, since the firstdielectric layer 3 is formed by the bias sputtering prior to thefilm-thickness adjustment with the milling, for example, the unevennessin the upper surface of the first dielectric layer 3 is made smaller.

The term “bias sputtering” implies a process of applying a bias voltageto a substrate during sputtering, and forming a film on the substrate bythe sputtering in the state where the bias voltage is applied to thesubstrate. The inventors of this application have discovered that theunevenness in the upper surface of the first dielectric layer 3 can bereduced by changing the bias voltage. FIGS. 4A to 4C are each aschematic sectional view illustrating the state of an interface betweenthe first and second dielectric layers 3 and 4. FIGS. 4A, 4B and 4Cindicate the results obtained at the bias voltage of V1, V2 and V3,respectively. Herein, there is a relationship of V1<V2<V3 among the biasvoltages.

As seen from FIGS. 4A to 4C, as the bias voltage increases, theunevenness in the upper surface of the first dielectric layer 3 isreduced.

The unevenness illustrated in FIGS. 4A to 4C is attributable to the factthat, as schematically illustrated in FIG. 5, the height of the firstdielectric layer 3 made of SiO₂ differs between a portion where a fingerelectrode 5 a of the IDT electrode 5 is present below and a portionwhere a gap between the electrode fingers 5 a and 5 a is present below.

Taking the above point into consideration, changes in the unevenness inthe upper surface of the SiO₂ film were observed while the bias voltagewas changed in forming the first dielectric layer 3. FIG. 6 plots theobserved result. A vertical axis of FIG. 6 represents the magnitude ofthe unevenness. With reference to FIG. 5, the magnitude of theunevenness implies a proportion of a difference H₀ between the height ofthe first dielectric layer above the center of the finger electrode 5 aand the height of the first dielectric layer above the center of the gapbetween the finger electrodes, i.e., a difference in height between aconcave portion and a convex portion, with respect to λ.

As seen from FIG. 6, the magnitude of the unevenness is smaller at thehigher bias voltage as illustrated in FIGS. 4A to 4C. Further, as seenfrom the result of FIG. 6, the magnitude of the unevenness is changedalong a curve of a quadratic function with respect to change of the biasvoltage. In other words, as the bias voltage increases, the magnitude ofthe unevenness is reduced and, in addition, a change amount of themagnitude of the unevenness with respect to the change of the biasvoltage is also reduced.

Accordingly, it is desirable to use a bias voltage range where not onlythe magnitude of the unevenness, but also a change rate of the magnitudeof the unevenness can be reduced. Though depending on the material used,etc., such a bias voltage range is desirably set to a range where aprocessing rate is reduced by about 10% or more in comparison with thatobtained at a film forming speed when the bias voltage is not applied.

However, if the bias voltage is too high, the film forming speed isreduced and a cost is increased. Accordingly, it is desirable to use avoltage range where the processing rate is not reduced to 50% or belowin comparison with that obtained at the film forming speed when the biasvoltage is not applied. In this preferred embodiment, as describedabove, after forming the first dielectric layer 3 by the biassputtering, the upper surface of the first dielectric layer 3 isflattened by the milling, for example. A process for the flattening isnot limited to the milling, and it may be carried out by suitable one ofother various methods, such as dry etching, CMP, and etching-back. Itshould be noted that, in this preferred embodiment, the flatteningprocess may not be carried out because the first dielectric layer 3 isformed by the bias sputtering and the unevenness in the upper surface ofthe first dielectric layer 3 is held small to begin with. Anyway, withthe flattening process, the unevenness in the upper surface of the firstdielectric layer 3 can be made smaller and a flatter surface can beobtained.

Another deposition method may be used instead of the bias sputtering. Inthat case, the magnitude of the unevenness is preferably held at about5% or less by carrying out the flattening process.

In the flattening process, after forming the first dielectric layer 3,the resonant frequency and the thickness of the obtained SiO₂ film aremeasured to determine an amount of SiO₂ to be removed.

FIG. 7 is a graph plotting the relationship between the magnitude ofunevenness of the first dielectric layer 3 and the resonant frequency ineach of states after the formation of the first dielectric layer 3 andafter the formation of the second dielectric layer 4 in this preferredembodiment. In FIG. 7, a mark ⋄ represents the resonant frequency afterthe formation of the first dielectric layer, and a mark □ represents theresonant frequency after the formation of the second dielectric layer 4.

As seen from FIG. 7, as the magnitude of unevenness in the upper surfaceof the first dielectric layer 3 increases, the resonant frequency afterthe formation of the first dielectric layer 3 lowers. More specifically,compared with the case where the magnitude of the unevenness is 0%,i.e., where the upper surface of the first dielectric layer 3 is flat,the resonant frequency lowers by about 10 MHz or more when the magnitudeof the unevenness exceeds about 5.0%, for example. Also, as themagnitude of the unevenness gradually increases in excess of about 5.0%,the resonant frequency further lowers. When the magnitude of theunevenness is about 9.8%, the resonant frequency lowers by about 40 MHz,for example.

Moreover, while the influence of the magnitude of the unevenness afterthe formation of the first dielectric layer 3 is relatively large, thefrequency change caused by the unevenness in the surface of the firstdielectric layer 3 after the formation of the second dielectric layer 4is relatively small. It is hence understood that the unevenness in thesurface of the first dielectric layer 3 greatly affects an amount of thefrequency change between before and after the formation of the seconddielectric layer 4.

On the other hand, FIG. 8 is a graph plotting the relationship betweenthe magnitude of the unevenness and a frequency tolerance after theformation of the second dielectric layer. Here, the term “frequencytolerance” implies the magnitude of frequency variation in the boundaryacoustic wave device 1. The graph of FIG. 8 is obtained by fabricatingmany boundary acoustic wave devices in which the magnitude of theunevenness has various values, determining a frequency variation, i.e.,a frequency tolerance, with respect to the design frequency, andplotting the determined result. As seen from FIG. 8, as the magnitude ofthe unevenness increases, a value of the frequency tolerance finallyobtained after the formation of the second dielectric layer alsoincreases.

At present, accuracy of about 0.5 MHz to several MHz is preferred as thefrequency tolerance in bandpass filters and antenna duplexers for a bandof about 0.5 GHz to about 3 GHz, which are primarily used in radiofrequency bands.

In the above-described preferred embodiment, the boundary acoustic waveresonator having the wavelength of about 1.9 μm, i.e., the frequency ina band of 2 GHz, is preferably provided. In that case, as seen from FIG.8, the magnitude of the unevenness is preferred to be about 5.0% or lessin terms of ratio to the wavelength for obtaining the frequencytolerance of ±1 MHz, for example.

In various preferred embodiments of the present invention, therefore,even when the thickness of the IDT electrode is as large as about 10% ormore of λ, the frequency tolerance can be made very small by settingH₀=H1−H2, which corresponds to the magnitude of the unevenness, to beabout 5.0% or less of λ. When the film thickness of the IDT electrode isless than about 10% of λ, the unevenness in the surface of the firstdielectric layer attributable to the formation of the IDT electrode isnot so large. As described above, however, when the film thickness ofthe IDT electrode is 10% or more, the unevenness in the surface of thefirst dielectric layer tends to become relatively large. For thatreason, various preferred embodiments of the present invention areparticularly effective in the boundary acoustic wave device of thethree-medium structure in which the film thickness of the IDT electrodeis preferably set to be about 10% or more of λ in order to reduce theresistance of the IDT electrode. It is hence to be noted that thecondition of setting the film thickness of the IDT electrode to be about10% or more does not imply a numerical restriction indicating a boundarybetween a range where the advantages achieved by various preferredembodiments of the present invention are obtained and a range where theadvantages achieved by various preferred embodiments of the presentinvention are not obtained.

Thus, one of the unique advantageous features of preferred embodimentsof the present invention is that, even when the film thickness of theIDT electrode is set to be about 10% or more of λ, the frequencytolerance in the three-medium structure can be remarkably reduced. Here,the point of setting the film thickness of the IDT electrode to be about10% or more of λ just represents a precondition for the boundaryacoustic wave device to which various preferred embodiments of thepresent invention are applied.

As the bias voltage increases, the film forming speed is reduced asdescribed above. In view of such a point, the manufacturing method maybe modified as follows. When the first dielectric layer 3 is formed, thebias voltage is adjusted such that the magnitude of unevenness in theupper surface of the first dielectric layer 3 is about 5% or more afterthe formation of the first dielectric layer 3, and that no voids areformed in the SiO₂. Further, after forming the first dielectric layer 3,the magnitude of the unevenness is reduced to about 5% or below by theabove-mentioned flattening process. In that case, the boundary acousticwave device having a sufficiently small frequency tolerance can beprovided according to preferred embodiments of the present inventionwithout significantly reducing the film-forming speed.

While the first dielectric layer 3 is preferably formed by the biassputtering in the above-described preferred embodiment, the firstdielectric layer in the boundary acoustic wave device of the presentinvention may be a deposited film, which is formed by some otherdeposition method than the bias sputtering.

While the IDT electrode 5 and the reflectors 6 and 7 are preferablyformed by the electron beam evaporation in the above-described preferredembodiment, they may be formed by some other suitable film-formingmethod, e.g., sputtering.

In the above-described preferred embodiment, the IDT electrode 5 ispreferably formed of the multilayer metal film containing Pt and AlCu asmain components. In the present invention, however, the IDT electrodemay be made of other suitable metals or may be formed of a single metalfilm.

When the IDT electrode 5 is formed of the multilayer metal film, it isdesirable that the multilayer metal film is primarily formed of a metalfilm made of Pt, Au, Cu, Ag, Al, W, or an alloy containing at least oneof those metals as a main component. The IDT electrode 5 thus formed canimprove the waveguide effect and reduce the resistance, thereby reducingthe loss.

Further, instead of the Ti film, a NiCr film or the like may be used asthe adhesive layer and/or the barrier layer.

While the first dielectric layer 3 is preferably made of SiO₂, thematerial of the first dielectric layer 3 is not limited to SiO₂. Inother words, the first dielectric layer 3 may be made of one of variousdielectric materials, such as silicon oxide, silicon oxynitride,aluminum oxide, aluminum nitride, and diamond-like carbon.

Also, the second dielectric layer 4 can be made of an appropriatedielectric exhibiting a higher acoustic velocity than the firstdielectric layer 3. Examples of such a dielectric include siliconnitride, silicon oxynitride, aluminum oxide, aluminum nitride, anddiamond-like carbon.

In addition, while the Ta₂O₅ film is preferably formed as the underlyingfilm of the IDT electrode 5 in the above-described preferred embodiment,the underlying film may be not formed. Further, the underlying film maybe made of, instead of Ta₂O₅, a material having the dielectric constantof 10 or more, such as TiO₂, Nb₂O₅, ZrO₂ or HfO₂.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. A boundary acoustic wave device comprising: a piezoelectricsubstrate; an IDT electrode located on the piezoelectric substrate; afirst dielectric layer arranged to cover the IDT electrode and made of adeposited film; and a second dielectric layer located on the firstdielectric layer; wherein assuming a wavelength of a boundary acousticwave to be λ, a normalized film thickness of the IDT electrode is about10% or more of λ, and a difference between a height of the firstdielectric layer, measured from an upper surface of the piezoelectricsubstrate, above a center of an electrode finger of the IDT electrodeand a height of the first dielectric layer, measured from the uppersurface of the piezoelectric substrate, above a center of a gap betweenadjacent electrode fingers is about 5% or less of the wavelength λ. 2.The boundary acoustic wave device according to claim 1, wherein thedeposited film of the first dielectric layer is a bias sputtered film.3. The boundary acoustic wave device according to claim 1, wherein anupper surface of the first dielectric layer is flattened.
 4. Theboundary acoustic wave device according to claim 1, wherein thepiezoelectric substrate is made of LiNbO₃ or LiTaO₃, and the firstdielectric layer is made of silicon oxide.
 5. The boundary acoustic wavedevice according to claim 4, wherein the second dielectric layer is madeof at least one dielectric material selected from a group consisting ofsilicon nitride, silicon oxynitride, aluminum nitride, aluminum oxide,and silicon.
 6. The boundary acoustic wave device according to claim 1,wherein the IDT electrode includes an electrode layer made of at leastone metal selected from a group consisting of Au, Ag, Cu, Pt, Ta, W, Ni,Fe, Cr, Mo, and Ti, and an alloy containing one of Au, Ag, Cu, Pt, Ta,W, Ni, Fe, Cr, Mo, and Ti, as a main component.
 7. A method ofmanufacturing a boundary acoustic wave device, the method comprising thesteps of: forming an IDT electrode on a piezoelectric substrate;depositing a dielectric material on the piezoelectric substrate to coverthe IDT electrode by a deposition method, thereby forming a firstdielectric layer; and forming a second dielectric layer on the firstdielectric layer; wherein assuming a wavelength of a boundary acousticwave to be λ, a normalized film thickness of the IDT electrode is about10% or more of λ, and a difference between a height of the firstdielectric layer, measured from an upper surface of the piezoelectricsubstrate, above a center of an electrode finger of the IDT electrodeand a height of the first dielectric layer, measured from the uppersurface of the piezoelectric substrate, above a center of a gap betweenadjacent electrode fingers is about 5% or less of the wavelength λ. 8.The method of manufacturing the boundary acoustic wave device accordingto claim 7, wherein the deposition method is bias sputtering.
 9. Themethod of manufacturing the boundary acoustic wave device according toclaim 7, further comprising the step of flattening a surface of thefirst dielectric layer after forming the first dielectric layer by thedeposition method.
 10. The method of manufacturing the boundary acousticwave device according to claim 7, wherein the step of flattening theupper surface of the first dielectric layer is carried out by at leastone process selected from a group consisting of milling, dry etching,CMP, and etching-back of the surface of the first dielectric layer.